What capture card? Not that it matters much...I had the same issue with FCP 6. Which is why I couldn't do a reliable INSERT EDIT, unless I went all the way to the commercial break (black)...or all the way until the end. But that was an earlier FCP 6...are you updated to 6.0.6?

Again...what capture card are you using? This for me was solved by updating to FCP 6.0.6, and the latest version of my capture card software. At the time I was running an AJA KONA 3...forget the software version of that one..